Activation of the C3b Receptor to Mediate Phagocytosis in Human Polyphonuclear Leukocytes: Role of Protein Kinase C and Calcium Flux

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Abstract

Polymorphonuclear neutrophilic leukoctes are one class of
white blood cells that function to phagocytise antigenic
material. In humans, this process can be mediated by two
distinct classes of proteinaceous cell surface receptors:
receptors for the Fc portion of immunoglobulin and receptors
for fragments of the third complement component, C3. While Fc
receptors constitutively mediate phagocytosis of IgG-coated
particles, C3 receptors are under regulatory control. Purtubation of neutrophil membranes with
tumor-promoting phorbol ester induces a functional alteration
of complement receptors rendering them capable of efficiently
mediating phagocytosis. Since
phagocytosis is a cytoskeletal dependent process, some
mechanism of signal transmission from receptors to elements of
the cytoskeleton is suggested. Recently, investigation of a group of membrane phospholipids has given considerable insight into a potential mechanism by
which the type one complement receptor, termed CR1, is
activated to mediate phagocytosis. This system, the polyphosphoinositide
system, is believed to transduce signals for a
variety of receptor systems. Turnover of the phosphoinositols
releases two agents, diacylglycerol and inositol trisphosphate,
both of which are now known to function as second messengers. Under physiologic conditions, diacylglycerol
functions in the plane of the membrane and directly activates
the enzyme protein kinase C, while inositol trisphosphate
induces calcium flux from the smooth endoplasmic reticulum. The current study
has been designed to elucidate, in part, the nature of a signal
transducing mechanism for CR1 mediated phagocytosis as
influenced by turnover of the polyphosphoinositides. Effects
of the known protein kinase C activating agents
sn-1,2-dioctanoylglycerol and sn-I-oleoyl-4-acetoylqlycerol
were studied for their capacity to induce CR1 internalization.
Additionally, since protein kinase C is a calcium dependent
enzyme, the effect of calcium flux was investigated. These
data suggest that complement receptors are activated to mediate
phagocytosis via action of protein kinase C.